vm Computer Architecture Lecture 16: Virtual Memory

vm.2 Review: The Principle of Locality °The Principle of Locality: Program access a relatively small portion of the address space at any instant of time. Example: 90% of time in 10% of the code Address Space0 2 Probability of reference

vm.3 Review: Levels of the Memory Hierarchy CPU Registers 100s Bytes <10s ns Cache K Bytes ns $ /bit Main Memory M Bytes 100ns-1us $ Disk G Bytes ms cents Capacity Access Time Cost Tape infinite sec-min Registers Cache Memory Disk Tape Instr. Operands Blocks Pages Files Staging Xfer Unit prog./compiler 1-8 bytes cache cntl bytes OS 512-4K bytes user/operator Mbytes Upper Level Lower Level faster Larger

vm.4 Outline of Today’s Lecture °Recap of Memory Hierarchy °Virtual Memory °Page Tables and TLB °Protection

vm.5 Virtual Memory? Provides illusion of very large memory – sum of the memory of many jobs greater than physical memory – address space of each job larger than physical memory Allows available (fast and expensive) physical memory to be very well utilized Simplifies memory management Exploits memory hierarchy to keep average access time low. Involves at least two storage levels: main and secondary Virtual Address -- address used by the programmer Virtual Address Space -- collection of such addresses Memory Address -- address of word in physical memory also known as “physical address” or “real address”

vm.6 Basic Issues in VM System Design size of information blocks that are transferred from secondary to main storage block of information brought into M, and M is full, then some region of M must be released to make room for the new block --> replacement policy which region of M is to hold the new block --> placement policy missing item fetched from secondary memory only on the occurrence of a fault --> fetch/load policy Paging Organization virtual and physical address space partitioned into blocks of equal size page frames pages reg cache mem disk frame

vm.7 Address Map V = {0, 1,..., n - 1} virtual address space M = {0, 1,..., m - 1} physical address space MAP: V --> M U {0} address mapping function n > m MAP(a) = a' if data at virtual address a is present in physical address a' and a' in M = 0 if data at virtual address a is not present in M Processor Name Space V Addr Trans Mechanism fault handler Main Memory Secondary Memory a a a' 0 missing item fault physical address OS performs this transfer

vm.8 Paging Organization frame P.A. Physical Memory 1K Addr Trans MAP page K unit of mapping also unit of transfer from virtual to physical memory Virtual Memory Address Mapping VApage no.disp 10 Page Table index into page table Page Table Base Reg V Access Rights PA + table located in physical memory physical memory address actually, concatenation is more likely

vm.9 Address Mapping Algorithm If V = 1 then page is in main memory at frame address stored in table else address located page in secondary memory Access Rights R = Read-only, R/W = read/write, X = execute only If kind of access not compatible with specified access rights, then protection_violation_fault If valid bit not set then page fault Protection Fault: access rights violation; causes trap to hardware, microcode, or software fault handler Page Fault: page not resident in physical memory, also causes a trap; usually accompanied by a context switch: current process suspended while page is fetched from secondary storage

vm.10 Virtual Address and a Cache CPU Trans- lation Cache Main Memory VAPA miss hit data It takes an extra memory access to translate VA to PA This makes cache access very expensive, and this is the "innermost loop" that you want to go as fast as possible ASIDE: Why access cache with PA at all? VA caches have a problem!

vm.11 Virtual Address and a Cache CPU Trans- lation Cache Main Memory VAPA miss hit data It takes an extra memory access to translate VA to PA This makes cache access very expensive, and this is the "innermost loop" that you want to go as fast as possible ASIDE: Why access cache with PA at all? VA caches have a problem! synonym problem: two different virtual addresses map to same physical address => two different cache entries holding data for the same physical address! for update: must update all cache entries with same physical address or memory becomes inconsistent determining this requires significant hardware, essentially an associative lookup on the physical address tags to see if you have multiple hits

vm.12 TLBs A way to speed up translation is to use a special cache of recently used page table entries -- this has many names, but the most frequently used is Translation Lookaside Buffer or TLB Virtual Address Physical Address Dirty Ref Valid Access TLB access time comparable to, though shorter than, cache access time (still much less than main memory access time)

vm.13 Translation Look-Aside Buffers Just like any other cache, the TLB can be organized as fully associative, set associative, or direct mapped TLBs are usually small, typically not more than entries even on high end machines. This permits fully associative lookup on these machines. Most mid-range machines use small n-way set associative organizations. CPU TLB Lookup Cache Main Memory VAPA miss hit data Trans- lation hit miss 20 t t 1/2 t Translation with a TLB

vm.14 Reducing Translation Time Machines with TLBs go one step further to reduce # cycles/cache access They overlap the cache access with the TLB access Works because high order bits of the VA are used to look in the TLB while low order bits are used as index into cache

vm.15 Overlapped Cache & TLB Access TLB Cache bytes index 1 K page #disp assoc lookup 32 PA Hit/ Miss PA Data Hit/ Miss = IF cache hit AND (cache tag = PA) then deliver data to CPU ELSE IF [cache miss OR (cache tag = PA)] and TLB hit THEN access memory with the PA from the TLB ELSE do standard VA translation

vm.16 Problems With Overlapped TLB Access Overlapped access only works as long as the address bits used to index into the cache do not change as the result of VA translation This usually limits things to small caches, large page sizes, or high n-way set associative caches if you want a large cache Example: suppose everything the same except that the cache is increased to 8 K bytes instead of 4 K: virt page #disp cache index This bit is changed by VA translation, but is needed for cache lookup Solutions: go to 8K byte page sizes go to 2 way set associative cache (would allow you to continue to use a 10 bit index) 1K way set assoc cache

vm.17 Fragmentation & Relocation Fragmentation is when areas of memory space become unavailable for some reason Relocation: move program or data to a new region of the address space (possibly fixing all the pointers) Internal Fragmentation: program is not an integral # of pages, part of the last page frame is "wasted" (obviously less of an issue as physical memories get larger) 0 1k-1... occupied External Fragmentation: Space left between blocks.

vm.18 Optimal Page Size Choose page that minimizes fragmentation large page size => internal fragmentation more severe BUT increase in the # of pages / name space => larger page tables In general, the trend is towards larger page sizes because Most machines at 4K-64K byte pages today, with page sizes likely to increase -- memories get larger as the price of RAM drops -- the gap between processor speed and disk speed grow wider -- programmers desire larger virtual address spaces

vm.19 2-level page table Seg 0 Seg 1 Seg bytes 256 P0 P255 4 bytes 1 K PA D0 D1023 PA Root Page Tables Data Pages 4 K Second Level Page Table x x x = Allocated in User Virtual Space 1 Mbyte, but allocated in system virtual addr space 256K bytes in physical memory

vm.20 Page Replacement Algorithms Just like cache block replacement! Least Recently Used: -- selects the least recently used page for replacement -- requires knowledge about past references, more difficult to implement (thread thru page table entries from most recently referenced to least recently referenced; when a page is referenced it is placed at the head of the list; the end of the list is the page to replace) -- good performance, recognizes principle of locality

vm.21 Page Replacement (Continued) Not Recently Used: Associated with each page is a reference flag such that ref flag = 1 if the page has been referenced in recent past = 0 otherwise -- if replacement is necessary, choose any page frame such that its reference bit is 0. This is a page that has not been referenced in the recent past -- clock implementation of NRU: page table entry page table entry ref bit last replaced pointer (lrp) if replacement is to take place, advance lrp to next entry (mod table size) until one with a 0 bit is found; this is the target for replacement; As a side effect, all examined PTE's have their reference bits set to zero. 1 0 An optimization is to search for the a page that is both not recently referenced AND not dirty.

vm.22 Demand Paging and Prefetching Pages Fetch Policy when is the page brought into memory? if pages are loaded solely in response to page faults, then the policy is demand paging An alternative is prefetching: anticipate future references and load such pages before their actual use + reduces page transfer overhead - removes pages already in page frames, which could adversely affect the page fault rate - predicting future references usually difficult Most systems implement demand paging without prepaging (One way to obtain effect of prefetching behavior is increasing the page size

vm.23 Summary °Virtual memory a mechanism to provide much larger memory than physically available memory in the system °Placement, replacement and other policies can have significant impact on performance °Interaction of Virtual memory with physical memory hierarchy is complex and addresses translation mechanisms must be designed carefully for good performance.